your situation is actually excellent for investors, because:
their money will not go towards paying your infrastructure bills or developing your service (perhaps only to expand it if demand increases)
BUT can be used towards pure customer acquisition (marketing, hiring sales and support personnel, etc...)
WHILE at the same time they have a very good case for your profitability, since with such small expenses, it is almost guaranteed (or at least the fail margin is considerably lower than usual) that you won't fail on expenses alone.
so the benefit FOR YOU is rapid expansion (marketing, sales, social media) with the money you get from investors (as in - getting more views from your target groups, therefore getting more business, therefore > profit).
basically, here you are thinking like the McDonald's brothers who opened up one restaurant it was a success and then they opened another. that was it.
then they sold out and the new buyer expanded the franchise to what it is today.
the brothers were happy with their two-restaurant expansion. with investors the (eventual) corporation could go global and acquire such a huge market it has today.
same for you, you may be happy with your current situation, but more money from investors means a bigger market share possibility.
other than that, using the money to BUY out or MERGE with other similar services is also a money well spent (especially if your investors can help with advice or actual takeovers/mergers). you may think your inventory is big, but it can always be bigger (example: amazon.com - watch http://www.infoq.com/presentations/vogels-amazon-platform).
also, if you can make use of adwords and similar programs (by hiring an expert to set it up) you are just exchanging money for more money. suppose the experts sets (and keeps optimizing) your campaigns so that each campaign spends (even) slightly less money than you get profit from the business it generates. in the case you have such a campaign, it makes sense to keep increasing the money to run the campaign (as long as it remains profitable) and you keep getting more and more profit from it.
if $10 campaign can bring $1 profit ($11 dollars total), then $1 million can bring $100,000 profit. this is usually called leverage, and can only be made if you have investors or bank assistance (in your case - investors are realistically more available than the bank).